1. Field of the Invention
The present invention relates to apparatus and process for making carbon black using slotted premix burners of a type not heretofore used in this field.
Flames are commonly characterized as premixed, or non-premixed, depending upon whether the fuel and oxidant have been blended into a substantially uniform mixture before being ignited. A substantially uniform mixture is defined as encompassing the mixtures of gaseous fuel with gaseous oxidant, vapors of normally liquid fuels with gaseous oxidant, and atomized droplets or mists of liquid fuels with oxidant gas. The art of premix burner design and operation is distinguished from that of non-premix burners in that with premix burners, (1) special attention must be given to the ranges of stability because of the dangers of "flashback", (2) it is necessary to cool the burner, or make it of ceramics or other highly refractory material since premix flames are inherently faster developing than non-premix flames, and (3) the shape of the flame must be controlled so that its intense heat is directed so as to avoid damage to associated equipment and to concentrate the heat where it is useful.
The stability of flame is a matter of great practical concern. If the flowing velocity of the reactants is too great, the flame becomes detached, or "blows off" and may be extinguished. Where the velocity of the oncoming combustible premix is marginally too high, the flame base may oscillate and be unstable. If on the other hand a premixed flame is fed at too low a velocity, the flame may "flash back" into the mixing zone upstream of the burner, causing overheating of the burner, and possibly a dangerous explosion. When the characteristics of the combustion system are such that the differences between blow-off and flashback conditions are too narrow, it is difficult to operate efficiently and safely. Therefore, a system is preferred which is versatile, that is, one which has a wide range between blow-off and flashback conditions.
The velocities of blow-off and flashback will vary with the temperature of the fuel-oxidant mixture, the ratio of fuel to oxidant, and the chemical character of the fuel itself, including its composition and its heat content. The relationship of air/fuel ratio to the velocities of blow-off and flashback are of special interest in the manufacture of carbon black, since the air/fuel ratio is a major variable in process control in this field.
To attain the objectives of controlled burning, it is necessary to consider the fundamentals of flame propagation. M. W. Thring, in The Science of Flames and Furnaces, John Wiley and Sons, disclosed there are essentially five mechanisms that may be used for igniting an inflowing combustible premix. These are conduction of heat, diffusion of hot species, preheat, radiation of heat, and recirculation of hot products from the downstream zone of fully established combustion to the point of ignition. The first four, individually and together, may permit stabilizing flame propagation of common premixtures such as methane and air with inflowing premix velocities in the low to medium range of about 1.5 to 30 feet per second. If a similar flame is to be held at velocities an order of magnitude higher, ignition will require the recirculation of hot products. Such higher velocities commonly occur in industrial burners.
It is well known that a non-streamlined body placed in a flowing fluid will generate recirculating flow downstream of the body and behind it. The recirculation is generated only against the surface of the non-streamlined body in the flow, along a line described as the intersection of the farthest laterally projecting edge of the body with a plane normal to the direction of flow. This line becomes the locale of the transfer of heat and chemically active species if used as a flame-holder in a combustor; it becomes, in effect, a "line of ignition" because the recirculation turns behind the interrupting body as the flow advances downstream, reverses direction, and reenters the flow near the surface of the intruding body, bringing with it the heat and chemically active substances necessary for igniting the oncoming fuel. As a result, the overall character of combustion as a system will depend significantly upon the ratio of the length of the "line of ignition" to the mass of flowing reactants. The ratio of the total length of the "line of ignition" to the mass of reactants flowing through the system is determined by the arrangement and number of non-streamlined devices placed in the stream for a given mass rate. For comparisons among burners, it becomes logical to use the ratio of the total length of line of ignition to the cross-sectional area of the orifices, or area of flow, with velocity as a parameter.
In one aspect, the present invention is related to the arrangement of the apparatus of a burner which makes it possible and practical, in a simple manner, to control the ratio of the length of "line of ignition" along non-streamlined bodies placed in the path of premixed combusting reactants to the mass of the flowing reactants, while retaining the proper spatial relationships downstream of the non-streamlined bodies to accommodate the recirculation eddies.
The invention is also concerned with the interactions of parallel flames, hereinafter discussed in connection with the slot to pitch ratio.
Another aspect of the invention is the simplifying of apparatus for producing high fineness carbon blacks by removing the necessity for separate combustion and reaction zones without losing the desired quality of the product.
Another aspect of the invention is improved adaptability of burners to variations in the characteristics of the combustion mixture.
Yet another aspect of the invention involves the adaptability of the burners of the invention in the manufacture of furnace carbon black by controlling the shape of the flame.
According to another aspect, the invention relates to the use in carbon black manufacture of premix slotted burners which efficiently remove back-radiated heat from the burner elements through the cooling effect of the inflowing premix, thereby permitting the burner elements to be made of low cost metal rather than more expensive refractory material.
There are two general categories of furnace carbon black used in the automotive tire industry: the so-called soft blacks, such as those used in tire carcasses; and the blacks which impart high abrasion resistance to rubber used in tire treads, commonly known in the trade as "tread blacks". The tread blacks are much finer than the carcass blacks, that is, the particles are much smaller. The typical soft, or carcass, black may have a surface area of thirty or forty square meters per gram. These blacks are made at low velocities and high ratios of oil to flowing gases. Blacks of the tread grades may have surface areas of from sixty to one-hundred-fifty square meters per gram, or even more. Tread blacks are made at higher velocities and lower ratios of oil to flowing gases than the carcass blacks. In recent years there has been a tendency to make the tread grades finer to allow more surface area for the interaction of the carbon and the rubber.
In the conventional furnace process for manufacturing carbon black, the hydrocarbon fluid fuel, commonly natural gas or fuel oil, is burned in a stream of process air furnished by a blower. The hot gases produced by the combustion of the fuel flow through a vessel, usually lined with refractory, and ordinarily of circular cross section. A feedstock oil, preferably highly aromatic, which is the chief source of carbon in the system, is injected into the flowing hot gases downstream of a point where the combustion of the fuel is complete. The oil feedstock must be vaporized as one step in the carbon forming process in order for the process to be successful. Vaporization is favored by high velocity of the hot gas stream, a high degree of turbulence, high temperature, low concentration of the oil, and high degree of atomization of the oil.
The feedstock oil vapor is carried by the hot gases formed by the combustion of the fuel, the fuel gases attaining temperatures of from about 2400.degree. F. to 3400.degree. F., varying with the methods used for controlling combustion. Radiant heat from the refractory, heat directly transmitting by the hot gases, high shear and mixing in the hot gases, and combustion of a portion of the oil by residual oxygen in the products of combustion all combine to transfer heat very rapidly to the feedstock oil vapors. Under these conditions, the oil molecules are cracked, polymerized and dehydrogenated, and progressively become larger and less hydrogenated until some reach a state such that they may be called nuclei of carbon. The nuclei grow in size, and at some stage there is aggregation of particles to form cluster-like agglomerates, known in the industry as "structure". At the completion of the process, the hot gases containing the carbon black are quenched to a temperature low enough to stop the reactions, and to allow the carbon black to be collected by conventional means.
The ultimate size of the particles depends, among other things, upon the concentration of the material from which they form. It is well-known, for example, that with a given furnace process, increasing the ratio of vaporized feedstock oil to the mass of flowing hot gases will produce larger particles and vice versa.
Several means have been devised by the industry for increasing the ratio of feedstock oil to the mass of hot flowing gases without losing fineness; that is, to maintain small particle size in spite of an increase of the ratio of feedstock oil to combustion gases. One method is to preheat the air furnished to the combustion zone, so that the addition of heat allows the rapid cracking of more oil. Another device is to increase the velocity of the flowing hot gases by a constriction in the reactor. This technique has been empirically established as a means of making small particle size, and is almost universally practiced in the manufacture of tread grade carbon blacks. High velocity, however, in conjunction with the high temperatures of the process, leads to problems of refractory deterioration in the constriction. Those skilled in the art are aware that only a little damage to the constriction is required to cause quality deterioration in the product.
Besides particle size, an important property of the tread grades of carbon black is what is known in the trade as "structure" discussed above. In the electron microscope, carbon black will appear as clusters of minute, imperfectly shaped spheroids fused together. The degree of clustering is called structure, and it is measured by a standard test of the American Society for Testing and Materials, Method D2414-72. In the test, the carbon absorbs dibutyl phthalate while being stirred in a standard mixing chamber until the torque on the mixing blades, generated by adding the dibutyl phthalate oil to the black, reaches a predetermined value. The greater the volume of dibutyl phthalate required to be mixed with the black to reach the torque prescribed, the greater the structure. This test is referred to as DBP absorption.
Structure may be classified as "temporary" or "permanent". When carbon black is first produced, it is a fluffy powder, with a high DBP value. Mechanically working the fluffy, high DBP carbon black causes some of its structure to break down. For example, a carbon black sample may have a DBP absorption value of about 130 when produced; then, when the pelleting machines work it into pellets for commercial use, the DBP absorption value may fall to about 115. More mechanical work on the carbon black will decrease its DBP value even further, until a point is reached at which further mechanical work done on it reduces the DBP absorption value insignificantly. A test referred to commonly as "24M4 DBP" is used to represent "permanent" structure, approximating the effects of milling the carbon black in rubber. The test is the DBP absorption on blacks which have been crushed four times at a pressure of 24,000 psi. It is published by the American Society for Testing and Materials as "Proposed Method B of D2414-72".
Another property of tread blacks which is significant as to its quality is porosity. Porosity is usually estimated by comparing the surface area of the carbon black measured by the adsorption of a small molecule with the surface area measured by a larger molecule. The rationale of the comparison is that the smaller molecule can make its way into pores or crevices in the surface which cannot be penetrated by larger molecules. Thus, if the two measurements are close together, there is relatively little porosity; but if they are far apart, there is more porosity. A surface which is less porous is more desirable in blacks to be used in the compounding of tread rubber.
Two commercial methods for measuring surface area are by nitrogen adsorption, standardized by the American Society for Testing and Materials as Method D-3037-76, and by iodine adsorption, standardized by the ASTM as method D-1510-76.
Another adsorption method for measuring surface area uses hexadecyltrimethylammonium bromide (abbreviated CTAB) which has a molecule too large to enter micropores which are accessible to nitrogen molecules. One porosity measure, then, is the nitrogen surface area minus the CTAB surface area. The smaller the difference, the less the porosity of the surface.
In addition to the adsorptive methods of estimating the fineness of carbon black, a tint test may be used. The tint test is a method of measuring the ability of a carbon black to cover the surface of a finely divided zinc pigment as compared to a standard carbon black. The method is standardized by the American Society for Testing and Materials, and is published as Method Number D-3265-76. The results of the test are such that higher numbers indicate greater fineness and vice versa.
2. Description of the Prior Art
The use of parallel slots in premix burners is known. U.S. Pat. No. 1,045,473 to Van Zandt, shows an externally fired cylinder with parallel slots for burning a premix of fuel gas and air, but with beveled edges on the pieces separating the slots, a practice contrary to the teachings of the present invention. U.S. Pat. No. 1,245,346 to Humphrey portrays a group of small slot burners spaced apart in such manner as not to be adaptable to the practice of the present invention. U.S. Pat. No. 1,717,667 to Curran claims parallel slots formed by ceramic pieces which are operated at high enough temperature to be thermal igniters, and secondary slots so oriented as to form a swirling combusting mass. Both of these techniques are contrary to the teachings of the present invention. The device of Geurink et al described in U.S. Pat. No. 1,830,393 is only partially a premix burner, as indicated by lines 14 ff., page 1, first column, and lines 23 ff., where the shape of the transverse channels is said to be designed to induce the inflow of secondary air. A slotted conical shape is portrayed in U.S. Pat. No. 2,018,582 to Theunissen, in which secondary air only, without gas, enters through the slots of the cone into a mixture of primary air and fuel gas. This sequence of admixing for combustion is contrary to the teachings of the present invention. The burner of Jaye, described in U.S. Pat. No. 2,573,144 is designed to entrain secondary air (col. 1, lines 26 ff.), and the patent teaches the apparatus for producing a bifurcated flame, and the advantageous use thereof. This patent also teaches a slot/pitch ratio of from 0.091 to 0.200 (col. 2, lines 9 ff.), while the present invention teaches the advantage of lower ratios. U.S. Pat. No. 2,621,722, line U.S. Pat. No. 1,045,473, teaches the use of an externally fired cylinder. U.S. Pat. No. 2,788,848 to Furczyk describes ceramic plates used to form slots, into which funnel shaped recesses are made for the mixing of the fuel and air. U.S. Pat. No. 3,035,633 to Palko describes an externally fired frusto-conical shape supplied with oil vapors from an evaporator which is an integral part of the burner, and with air premixed with the vapors. U.S. Pat. No. 3,277,948 to Best portrays a slotted burner, the ceramic elements of which form the slots, designed to operate with the ceramic elements at incandescence, contrary to the present invention, one advantage of which is the cooling of the parts which form the slots. In FIG. 2 of Hine's U.S. Pat. No. 3,285,317 the slots are shown as discontinuous; and the burner strip is dished to prevent resonance. Resonance under certain conditions is a characteristic of the burners of the present invention. In U.S. Pat. No. 3,361,367 to Hein et al the parallel slots are of unequal length and have no specified slot/pitch ratio. In this patent there is special provision for reducing the pressure at the ends of the burner to prevent blow-off, a clear indication that instability would be a characteristic of this arrangement of slots without the special provision.
The stabilizing of flames by various schemes which involve the inducement of recirculation, or the use of vortex motion, are known. One common method is to suddenly enlarge a cylindrical conduit carrying the combustible mixture to a larger diameter, producing a step in the wall which causes turbulent recirculation.
Another scheme for flame stabilization is to create a swirl by the injection of a portion of the reactants tangentially, as illustrated by U.S. Pat. No. 3,187,799 to Nesbitt. Carried to its ultimate, the use of swirl is illustrated in U.S. Pat. No. 3,490,869 to Heller, in which the total mass of oxidant and fuel enters tangentially, forming one great vortex. It is noted in this case also, that the flame sweeps a hot refractory surface, taking advantage of surface effects. Spin vanes may be employed as in U.S. Pat. No. 3,254,846 to Schreter et al, and German Pat. No. 2418274, both of which treat of burning liquid fuel. A single deflector, which amounts to a non-streamlined body placed in the flow, may be used in U.S. Pat. Nos. 3,009,787 to Ruble, and 3,408,165 to Hinson. Another common technique is portrayed in British Pat. No. 1,359,216; a mixture of air and fuel gas passes through perforations, shown as circular, in a metal flameholder, the solid portions of which, between the perforations, act as non-streamlined objects in the path of the flow of combustion mixture. There are numerous instances of gas being injected through orifices or slots into air; for example Morgan's U.S. Pat. No. 3,614,283.
In the field of the use of burners for the production of furnace carbon black, typical assemblies are shown in the following U.S. Pat. Nos.: Deland 3,490,870; Latham 2,976,127; Krejci 2,865,717; Williams 3,060,003; Latham 3,256,065; Heller 3,490,869.
The doctrine and practice are well established in patents issued, and in the industry, that in the manufacture of high fineness carbon black the combustion zone and the reaction zone are separated. For example Wrigley in U.S. Pat. No. 2,780,529 shows in FIG. 1 what are characterized in column 2, lines 41-43 as a combustion zone and a reaction zone. Also, Heller in U.S. Pat. No. 3,490,869, column 2, lines 64 ff., discusses this subject.
In U.S. Pat. No. 3,026,185 Takewell et al state at column 2 line 23 ff that reducing a constriction diameter to half its original value increased the surface area of carbon black from 80 square meters per gram to 200 square meters per gram.
Latham in U.S. Pat. No. 3,256,065 discusses at column 1, third paragraph, the effects of turbulence in obtaining rubber reinforcing grades of carbon black. Turbulence is known to be a function of velocity. At column 2, lines 15 ff Latham states, "The advantages enumerated above are primarily realized by employing a Venturi section in a tubular carbon black reactor--." The primary effect of a Venturi is to increase greatly the velocity of flow. Numerous patents portray constrictions designed to increase velocity in carbon black reactors.
It is apparent that a new process and apparatus which is able to match the quality of carbon black from conventional process and apparatus at half or one-third the velocity required by the conventional process with other conditions essentially the same, is inherently different.